EXTRACTION OF RARE EARTH METALS FROM NdFeB USING  SELECTIVE SULFATION ROASTING

ABSTRACT

Sulfuric acid baking allows for the selective extraction of rare earth metals from scrap Nd 2 Fe 14 B magnets. This process has the advantage of making the high-value rare earth metals water soluble while converting the contained iron, around 70% of the magnet&#39;s mass, into stable and insoluble iron (III) oxide, eliminating the need for separating and precipitating iron from solution. The process also has the advantage of safely disposing of metalworking fluid contained in rare earth magnet machine waste, a troublesome contaminate from technical and environmental perspective.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority and the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/047,381 filed on Sep. 8, 2014, which is incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The disclosure relates to the extraction of rare earth metals from magnet scrap or used magnets using a sulfuric acid baking process.

BACKGROUND

Around one quarter of the world's rare earth metals are used in rare earth magnets. With sintered magnets, up to 30% of the starting magnetic alloy is lost to scrap and machine waste. This scrap material often becomes contaminated with oxygen, grinding media, and other metals which prevent it from being able to be recycled back into magnet production. Conventional extraction of these rare earth metals requires the roasting of the magnet waste or used magnets, which produces oxides. These oxides must then be leached with mineral acids to obtain the rare earth metals and the solutions treated to obtain mix or impure rare earth oxides, oxalates or hydroxides. These must be purified further before the contained metals can be reduced.

Another issue with the rare earth magnet swarf (fine chips of rare earth magnets that are produced during machining) waste stream is that it contains spent metalworking fluid, a troublesome contaminate from both technical and environmental perspectives. These are usually organic based fluids that need to be removed prior to subsequent processing.

The conventional recovery method involves the leaching of the roasted magnet waste with an acid leach solution for several hours, at a low pulp density, possibly using an elevated temperature leach. Some of the contained iron goes into solution as well, consuming acid and necessitating its removal downstream. The handling of large volumes of acid leach solution adds to the operating costs of a process, as does the residence time necessary for dissolution. Also, the removal and disposal of iron from solution represents a significant burden on a process.

SUMMARY

The present invention is directed towards a process for extracting rare earth metals from Nd₂Fe₁₄B magnet scrap, or used magnets, using a sulfuric acid bake, which improves upon the conventional extraction method by eliminating the need for separating and precipitating iron from the waste solution and also by safely disposing of the metalworking fluid contained in the waste by combustion.

Sulfuric acid baking allows for the selective extraction of rare earth metals from scrap or used Nd₂Fe₁₄B magnets. The sulfuric acid bake works by taking advantage of the higher thermal stabilities of the rare earth sulfates compared to that of iron. As the mixture is heated/roasted under oxidizing conditions at temperatures between 200° C. and 900° C., neodymium sulfate becomes the dominate rare earth phase while the iron is converted into iron (III) oxide.

This process has the advantage of making the high-value rare earth metals water soluble while converting the contained iron, around 70% of the magnet's mass, into stable and insoluble iron (III) oxide, eliminating the need for separating and precipitating iron from solution. The process also has the advantage of safely disposing of metalworking fluid contained in rare earth magnet machine waste, a troublesome contaminate from technical and environmental perspective. During the oxidizing sulfuric acid bake, the metalworking fluid is consumed via combustion, removing it from the waste stream.

An aspect of the invention is a method of extracting rare earth metals from rare earth magnet materials. The rare earth magnet material includes at least one rare earth metal and at least one other metal. Following preparation, the rare earth magnet material is mixed with an acid and water to form a slurry, which can be achieved by the addition of about 3 mL of liquid per gram solid feed. The material is reacted under oxidizing conditions at a temperature between about 200° C. and about 900° C., forming a baked mixture. The baked mixture is leached in water at a solid to liquid ratio of about 100 g solids per about 1 L water -forming a liquid leaching product comprising at least one soluble rare earth ion and a solid leaching residue comprising at least one other metal as an insoluble metal oxide. The solid leaching residue is removed using any suitable method, including thickening and filtration. The rare earth ions are then separated from the liquid leaching product by precipitation.

An aspect of the invention is a method for extracting at least one rare earth metal from a rare earth scrap material. The method includes removing a coating from the rare earth scrap material to produce a bare rare earth scrap material. The material includes at least one rare earth metal and one other metal. The bare rare earth scrap material is comminuted or crushed to produce a comminuted material. The comminuted material is mixed with an acid and water to form a slurry. The slurry is roasted under oxidizing conditions at temperatures between about 200° C. and about 900° C. The baked mixture is leached in an aqueous solution to form a liquid leaching product comprising at least one soluble rare earth ion, and a solid leaching residue comprising at least one other insoluble metal oxide. The rare earth ion is separated from the liquid leaching product by precipitation.

An aspect of the invention is a method for extracting rare earth metals from a rare earth containing material. The coating is removed from the rare earth scrap material to produce a bare rare earth scrap material. The bare rare earth scrap material is comminuted or crushed to produce a comminuted material. The comminuted material is mixed with an acid and water to form a slurry. Residual moisture is evaporated from the slurry to form an evaporated slurry. The evaporated slurry is roasted under oxidizing conditions at temperatures between about 200° C. and about 900° C. to form a baked mixture and an off gas. The off gas is scrubbed to form at least one of sodium sulfate and sodium sulfite. The baked mixture is leached in water to form a liquid leaching product comprising at least one soluble rare earth ion, and a solid leaching residue comprising at least one other insoluble metal oxide. The rare earth ion is precipitated from the liquid leaching solution. The precipitating agent is at least one of sodium sulfate and oxalic acid and at least a portion of the sodium sulfate is recycled from the off gas scrubbing step.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a flow schematic depicting an embodiment of the sulfuric baking extraction process;

FIG. 2 depicts a flow diagram of a preliminary acid bake water leach process in one embodiment of the invention;

FIG. 3 depicts the Gibbs Energy Minimization Graph for the sulfuric acid, iron, oxygen gas, and neodymium system; and

FIG. 4 depicts the Gibbs Energy minimization graph of the Fe, Nd, H₂SO₄, and O₂ system.

DETAILED DESCRIPTION

The present invention is directed to a method to recover rare earth elements from rare earth containing materials, including rare earth magnet scrap and used rare earth magnets.

The process of the present invention is much more efficient and economic than other proposed or used methods. Dissolution of the rare earths is much faster and can be done in much milder conditions when compared to the conventional acid leaching method, resulting in lower operating costs. Furthermore, the process of the present invention utilizes the ability to convert scrap or magnets into water soluble rare earth sulfates while keeping the iron in a water insoluble form. It also can recover the boron in a separate stream. The alternative, conventional method or oxidizing roasting and acid leaching dissolves a portion of the iron with the rare earths leading to much greater sulfuric acid consumption and much more difficult selective separation of the dissolved rare earth ions. The iron in the solution must then be removed by a separate processing step, increasing the costs.

An aspect of the invention is a method of extracting rare earth metals from rare earth magnet materials. The rare earth magnet material includes at least one rare earth metal and at least one other metal. The rare earth magnet material is mixed with an acid and water to form a slurry, which can be achieved by the addition of 3 mL of liquid per gram solid feed. The material is reacted under oxidizing conditions at a temperature between about 200° C. and about 900° C., forming a baked mixture. The baked mixture is leached in water, at a solid to liquid ratio of about 100 g solids per 1 L water forming a liquid leaching product comprising at least one soluble rare earth ion and a solid leaching residue comprising at least one other metal as an insoluble metal oxide. The solid leaching residue is removed using any suitable method, including thickening and filtration. The rare earth ions are then separated from the liquid leaching product by precipitation.

The rare earth magnet materials can be magnet scrap or used magnets. The other metal may include at least one of iron, neodymium, dysprosium and praseodymium.

In one embodiment, the slurry can include at least about 1.5 grams of the acid per about 1 gram of rare earth metal bearing material. The acid can be sulfuric acid. The range of acid addition can vary from about 1 to 2 grams per gram of rare earth bearing material depending on the solid elemental analysis. The amount of acid can vary depending on the amount metallic elements involved.

The slurry can be dried to remove residual moisture at low temperatures (room to about 100° C.) for between about 30 minutes to about 24 hours. The residual moisture can be removed by placing the material into a heating system, such as a furnace, a kiln, a rotary kiln, or multiple hearth furnace. In some embodiments, the residual moisture can be removed by subjecting the slurry to room temperature for between 30 minutes to about 24 hours. The goal of removing the residual moisture is to minimize or avoid flash boiling of the mixture. Iron oxide and neodymium sulfate can be formed following the baking step.

In another embodiment, the baking of the slurry may be performed in either a rotary kiln or a multiple hearth furnace. The baking step can occur for between about 15 minutes to about 2 hours. Following the bake, the roasted solids can be sent to the leaching circuit, where water can be used to selectively leach the rare earth metals.

The leaching step can be performed in a stirred tank reactor. The leaching can occur for a period of between about 15 to 60 minutes. The solid to liquid ratio during the leaching step can be fairly high, between about 50 grams solids per liter water to about 200 g/L. The rare earths can be converted to soluble sulfates and the iron contained in the solution can be converted to hematite. After leaching, the solid iron oxides can be separated from the leach liquid and the iron oxides are sent to tailing disposal or sold as powdered hematite product.

Components in the liquid from the leaching step can be separated into streams. In an embodiment, the precipitation of the rare earth ions can be performed with sodium sulfate (which can be produced as a byproduct of the off-gas purification or purchased commercially) and/or oxalic acid as well as a number of other precipitation agents. The sodium sulfate can be used in a first step of the precipitation, but can leave a fraction of the heavy rare earths in solution. Oxalic acid, when added to rate earth solutions, results in the formation of insoluble rare earth oxalates. Combining these precipitation agents can be used to form a rough separation between the light cerium group and the heavy yttrium group of the rare earth elements. In some embodiments, the precipitation can include a solvent extraction stage prior to precipitation, allowing for separate streams containing neodymium, praseodymium and dysprosium.

The leaching solution can then be separated. Solvent extraction can be used to separate the individual contained rare earths, neodymium, praseodymium, and dysprosium, into separate streams. In some embodiments, a single precipitation step using only oxalic acid as the precipitating agent can be used. The leaching liquid can be recycled, while taking out a small fraction, between approximately 0.1% to about 5%, to curtail the buildup of impurities.

Off gas from the bake step can be sent to a gas cleaning apparatus. Volatile compounds, such as boron compounds, can be converted to an acid, such as boric acid, by collection in a sodium hydroxide-water scrubber solution. Compounds including SO₂, can be scrubbed with caustic solutions, such as sodium hydroxide, to form sodium sulfite or sodium sulfate. The mixture can be mixed with a calcium source material, such as lime, to produce calcium sulfate in a dual alkali process. Sodium hydroxide can be regenerated and the solid calcium sulfate can be disposed. In some embodiments, the sodium sulfate can be fed into an electro-dialysis unit to convert the sodium sulfate to sodium hydroxide and sulfuric acid, both of which can be re-used in the process.

An embodiment of the invention is the separation of the rare earth material for processing. The rare earth metal material can be prepared by removing coatings from the surface of the material. By way of example only, if the magnets contain an aluminum coating, the coating can be removed by leaching the coating in a leaching solution. The leaching solution can include a solvent, such as NaOH, and, water at varying concentrations. For other coatings, such as nickel coatings, the de-coating can be accomplished using a strip solution particular for the metal. After the coating has been removed, the rare earth containing materials can be comminuted or crushed into a powder. The powder can have an average diameter of less than about 270 mesh.

An embodiment of the invention is illustrated in FIG. 1. First, scrap rare earth magnets are de-coated. For aluminum coatings, the de-coating is accomplished by leaching the magnets in a NaOH water solution. For nickel coatings, the de-coating is accomplished by a commercially available nickel plating strip solution. Following the de-coating, the magnets are crushed into a powder via traditional mineral processing equipment. A finer powder is preferred over a coarse powder. This powder may be then treated directly or mixed with swarf and is fed into a mixture with sulfuric acid of rare earth bearing material and water, creating a slurry. FIG. 2 also illustrates the preliminary acid bake water leach flowsheet of an embodiment of the invention. The material can be dried to remove residual moisture or fed directly into a furnace.

The mixture is then fed into rotary kiln and baked for a set time and temperature dependent on the equipment and feed. The time and temperature can be determined with the aid of FIG. 3. FIG. 3 illustrates the Gibbs energy minimization graph for the H₂SO₄, Fe, O₂, B, and Nd system. FIG. 4 illustrates the Gibbs Energy minimization graph of the Fe, Nd, H₂SO₄, and O₂ system. FIG. 3 and FIG. 4 both illustrate that at temperatures between about 200° C. and about 900° C., the rare earth sulfate becomes the dominant phase, while the iron is converted to iron (III) oxide, iron (II, III) oxide, or hematite. While the exact time and temperature can be adjusted based on the requirements of the equipment and feed, and would be understood by one skilled in the art, the conversion should take place in a relatively short period of time.

The off-gas from the rotary kiln is then sent to gas cleaning in which any present volatile boron compounds can be converted into boric acid by collection in a sodium hydroxide-water scrubber solution while any present SO₂ can be scrubbed with caustic solution, NaOH, to form sodium sulfite or sodium sulfate. In one embodiment, this may be mixed with lime and converted to calcium sulfate, in a dual alkali process. In this dual alkali system, the NaOH is regenerated, and the solid calcium sulfate may be disposed of In another embodiment, the sodium sulfate can be fed into an electro dialysis unit, converting it into NaOH and sulfuric acid, both of which can be re-used in the process.

The roasted solids from the rotary kiln containing the rare earth as sulfates and iron oxide can be sent to a water leaching step and combined with water as illustrated in FIGS. 1 and 2. The water can be make up water. Recycled liquid can also be combined in the water leaching step. Solids are separated from liquids in a first solid/liquid separation. The solids, which contain iron oxides and/or hematite, can be sent to a tailing pond while the liquid, which contain rare earths, can be sent to a first stage precipitation. In the first stage precipitation, the liquids can be combined with sodium sulfate. Solids are separated from the liquids in a second solid liquid separation. The solids can contain neodymium, and praseodymium, while the liquids can be further processed in a second stage precipitation. The solids can further be treated to isolate the neodymium from the praseodymium. In the second stage precipitation, the liquids from the second solid/liquid precipitation can be combined with oxalic acid. In a third solid/liquid separator, the solids (dry oxalate) can be removed from the liquid. The liquid can be recycled. In some embodiments, a small bleed of the liquid can be removed from the cycle.

EXAMPLES Example 1

A magnet from an electric motor was removed and its aluminum coating stripped with NaOH solution. It was then crushed to a 80% passing size of 270 mesh. It was found to have a composition (by weight) of:

-   58.9% Iron -   16.3% Neodymium -   4.5% Praseodymium -   8.2%Dysprosium -   2.0% Cobalt -   1.7%Boron

The crushed material was combined with 2 ml of water and 1 ml of sulfuric acid per gram of magnet powder, mixed, and allowed to dry at room temperature. The mixture was then broken up and placed in a furnace at 700° C. for a period of 45 minutes. After it had been taken out and allowed to cool, it was mixed with water and leached for 60 minutes. The mixture was then filtered and washed. It was found that the recovery of metals to solution were 98.6% of the neodymium, 99.4% of the praseodymium, 98.7% of the dysprosium, 80.9% of the cobalt, 22.2% of the boron and 0.5% of the iron. The remainder of the contained metal values remained in the leach residue, composed mainly of iron (III) oxide.

Ranges are set forth in the specification. One skilled in the art would understand that any sub-range within the ranges or any particular value within the range would be suitable for use.

The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiment described hereinabove is further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art. 

What is claimed is:
 1. A method for extracting rare earth metals from a rare earth containing material, comprising: mixing the rare earth containing material with an acid and water to form a slurry, wherein the rare earth containing material comprises at least one rare earth metal and at least one other metal; roasting the slurry under oxidizing conditions at temperatures between about 200° C. and about 900° C. to form a baked mixture and an off gas; leaching the baked mixture in an aqueous solution to form a liquid leaching product comprising at least one soluble rare earth ion, and a solid leaching residue comprising at least one other insoluble metal oxide; and separating the at least one soluble rare earth ion from the liquid leaching product by precipitation.
 2. The method of claim 1, wherein said slurry is comprised of at least about 1.5 gram of acid per 1 gram said rare earth bearing material.
 3. The method of claim 1, wherein the acid is sulfuric acid.
 4. The method of claim 1, wherein said precipitation is performed with oxalic acid as the precipitation agent.
 5. The method of claim 1, wherein said precipitation comprises a first a solvent extraction stage which separates individual rare earths into different process streams prior to precipitation
 6. The method of claim 1, wherein said baking is performed in a rotary kiln or a multiple hearth furnace.
 7. The method of claim 1, wherein the baked mixture comprises at least one rare earth sulfate
 8. The method of claim 1, further comprising preprocessing the rare earth containing material.
 9. The method of claim 8, wherein the preprocessing comprises removing a coating from the rare earth containing material.
 10. The method of claim 8, wherein the preprocessing comprises comminuting the rare earth containing material.
 11. The method of claim 1, wherein the rare earth containing material is selected from the group consisting of a rare earth magnet, a rare earth scrap, a rare earth swarf, and combinations thereof.
 12. The method of claim 1, wherein the other metal is at least one of iron, neodymium, dysprosium and praseodymium.
 13. The method of claim 1, further comprising evaporating excess moisture from the slurry prior to the roasting step.
 14. The method of claim 1, further comprising removing at least one volatile material from the off gas.
 15. The method of claim 1, further comprising scrubbing the off gas to remove SO₂ and form at least one compound of sodium sulfate and sodium sulfite.
 16. A method for extracting at least one rare earth metal from a rare earth scrap material, comprising: removing a coating from the rare earth scrap material to produce a bare rare earth scrap material; comminuting the bare rare earth scrap material to produce a comminuted material; mixing the comminuted material with an acid and water to form a slurry, wherein the comminuted material comprises at least one rare earth metal and at least one other metal; roasting the slurry under oxidizing conditions at temperatures between about 200° C. and about 900° C. to form a baked mixture; leaching the baked mixture in an aqueous solution to form a liquid leaching product comprising at least one soluble rare earth ion, and a solid leaching residue comprising at least one other insoluble metal oxide; and separating the at least one soluble rare earth ion from the liquid leaching product by precipitation.
 17. The method of claim 16, wherein the coating is an aluminum coating, and wherein the coating is removed by subjecting the rare earth scrap material to a leaching solution comprising sodium hydroxide.
 18. The method of claim 16, wherein the coating is a nickel coating, and wherein the coating is removed by subjecting the rare earth scrap material to a nickel striping solution.
 19. The method of claim 16, wherein the acid is sulfuric acid.
 20. A method for extracting rare earth metals from a rare earth containing material, comprising: removing a coating from the rare earth scrap material to produce a bare rare earth scrap material; comminuting the bare rare earth scrap material to produce a comminuted material; mixing the comminuted material with an acid and water to form a slurry, wherein the comminuted material comprises at least one rare earth metal and at least one other metal; evaporating residual moisture from the slurry to form an evaporated slurry; roasting the evaporated slurry under oxidizing conditions at temperatures between about 200° C. and about 900° C. to form a baked mixture and an off gas; scrubbing the off gas to form at least one of sodium sulfate and sodium sulfite; leaching the baked mixture in water to form a liquid leaching product comprising at least one soluble rare earth ion, and a solid leaching residue comprising at least one other insoluble metal oxide; and precipitating the at least one soluble rare earth ion from the liquid leaching, wherein the precipitating agent is at least one of sodium sulfate and oxalic acid, wherein at least a portion of the sodium sulfate is formed by scrubbing the off gas. 